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Date: Sat, 2 Dec 2000 20:08:32 -0500
From: Christopher Faylor <cgf@redhat.com>
To: cygwin <cygwin@cygwin.com>
Subject: Re: Why does scp leave ssh running? -- select() never returns
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In-Reply-To: <3A271DCD.99BDDBF5@delcomsys.com>; from wpd@delcomsys.com on Thu, Nov 30, 2000 at 10:41:01PM -0500

Sorry for the delay in responding.

On Thu, Nov 30, 2000 at 10:41:01PM -0500, Patrick Doyle wrote:
>> If that is the case, would anybody mind if I posted a (simple) patch to
>> openSSH code which added a timeout to the call(s) to select()?
>>
>> (Note, I haven't tried this patch yet -- I just thought of it ...
>
>Status update... 
>
>Adding the timeout to the call to select() did not solve the problem
>(which is consistent with Christopher's assertion that "The only way to
>trigger the EOF is to actually read from the pipe".  Actually, I wasn't
>too keen about solving the problem by hacking openSSH; it would just
>mean that the problem had to be solved again and again for other
>packages.
>
>So now what?  Does anybody have any suggestions for other things that I
>could try?  I would prefer to solve this problem in cygwin1.dll, but I
>am, by no means, an expert windows programmer, so right now, I find
>myself just flailing.
>
>The two ideas I have so far are:
>1) Switch from using anonymous pipes to using named pipes with unique
>names.  According to the documentation on the MSDN web site, this is how
>anonymous pipes are implemented in W2K and NT.  Maybe this will solve
>the problem, maybe it won't.  Maybe it will allow me to call a function
>which will let me know if the other end has closed the pipe.  (Most of
>the functions I have found on the MSDN web site are specific to named
>pipes).  Maybe it won't.

You can't create named pipes on Windows 95, unfortunately.

>2) Change the thread_pipe to call ReadFile() (reading one byte) instead
>of PeekNamedPipe() and save the result for the next call to read.  This
>has the advantage that it would probably work (based on Christopher's
>comment and my own experimentation with read() vs. select()), but it
>looks like a heck-of-a-lot of work (read: prone to bugs) to solve such a
>stupid problem.  Actually, suggestion #1 suffers from this con as well.

Your analysis is correct, IMO.  I've been mulling this over for a couple
of years and have never come up with a satisfactory solution.  Even the
thread method is prone to difficulties.  If you are blocked on a read of
a pipe, terminating the thread cleanly becomes problematic.

It's been some time since I mentioned that "I hate Windows 9x."  This seems
like a good place to bring this up.

>Any suggestions, comments, retorts, complaints about me cluttering up
>your email box with trivialities that you don't care about, would be
>welcome, er... probably not the last one.

I would be really happy if someone had a creative solution to this problem.
It would be nice to fix this in the Cygwin DLL.

cgf

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